1. Field of the Invention
The present invention relates to a film capacitor and a method of making the same.
2. Description of the Prior Art
The advent of the age of electronic engineering in various fields has emphasized the need to reduce the size, weight and price of electronic component parts while increasing the performance of these electronic component parts. This includes film capacitors and various attempts have hitherto been made to miniaturize a film capacitor in the form of a chip. Research has also been made to improve the dielectric strength and the moisture resistance of the film capacitor for the purpose of increasing the performance thereof.
One of the major causes of lowering both of the dielectric strength and the moisture resistance of the film capacitor appears to be the inclusion of air in between the neighboring laminae, for example, between a dielectric film and a film electrode, or between the dielectric film and a metal film, when the dielectric film and the film electrode are laminated one above the other or when the metal film is formed on the dielectric film to provide a metalized film. The presence of air gaps in the laminated body tends to allow water to intrude into the air gaps, eventually corroding some of the electrodes to such an extent as to result in a corona discharge accompanied by deterioration of films and electrodes. The consequence is that both the operating characteristics and the performance of the film capacitor are lowered.
In order to avoid the above discussed problems, an attempt has been made to fill up the air gaps with an electrically insulating material such as epoxy resin, polyurethane resin, oil or wax, thereby to improve the moisture resistance.
FIGS. 6a to 6e illustrate the sequence of making an example of the prior art film capacitors, so-called impregnated film capacitors, in which the electrically insulating material, for example, epoxy resin or polyurethane resin, is utilized to fill up the air gaps present in the laminated body, reference to which will now be made for the discussion of the prior art method of making the impregnated film capacitor.
In these FIGS. reference numeral 61 designates one-side metalized films each comprising a metallic foil 61a which eventually forms an electrode and formed on one of opposite surfaces of a respective film 61b by the use of, for example, a vacuum deposition technique. Reference numeral 62 designates a film gap defined between every other film; reference numeral 63 designates a marginal area which is a non-metalized area; and reference numeral 65 designates a film capacitor element.
According to the prior art, the one-side metalized films 61 are coiled or laminated while displaced alternately in a direction widthwise thereof to form alternating film gaps between every other film, thereby to provide the film capacitor element 65 as shown in FIG. 6a. Alternatively, the film capacitor element 65 may be formed by coiling or laminating a plurality of one-side metalized films of relatively large width embossed or otherwise perforated, and then by cutting a portion of each one-side metalized film which corresponds in position to one end thereof from which an electrode is drawn.
Then, as shown in FIG. 6b, opposite end electrodes 66 are formed at the opposite ends of the one-side metalized films, or the film capacitor element 65, in electrically connected relation with the electrodes 61a by the use of any known spray coating technique. The resultant capacitor element having the opposite end electrodes 66 is shown by 67 in FIG. 6c.
The capacitor element 67 is subsequently cut into a plurality of capacitor chips 67 as shown in FIG. 6d, which chips 67 are subsequently transported to a vacuum chamber where they are immersed in bath a containing epoxy resin or polyurethane resin to allow the epoxy or polyurethane resin to penetrate into air gaps or clearances formed during the lamination of the capacitor films 61. Thereafter, the capacitor chips 67 having been immersed in the resin bath are removed from the vacuum chamber and, in an atmospheric environment, the resin is allowed to cure to form a resin layer 68 between each neighboring one-side metalized film 61 as shown in FIG. 6e so that the resultant film capacitors can have an increased moisture resistance and an increased dielectric strength.
However, it has been found that the prior art method hereinabove described still has the following problems.
(1) Since the end electrodes have been formed prior to the capacitor element being immersed in the resin bath, the resin does not uniformly penetrate into the air gaps formed during the lamination of the films. Because of this, some or all of the capacitor chips still contain air gaps and, therefore, neither sufficient moisture resistance nor sufficient dielectric strength can be obtained.
(2) Since the end electrodes formed by the use of the metal spray coating technique contain voids, the resin tends to penetrate into the voids in the end electrodes and also to cover the end electrodes. Because of this, where lead wires are required to be welded to the respective end electrodes, the weldability tends to be lowered, or where lead wires are required to be soldered to the respective end electrodes, the solderability tends to be lowered because solder deposits fail to stick to the end electrodes.
(3) The method step during which the resin is filled into the air gap between each neighboring films requires a relatively long period of time, for example, about 12 hours.